BOTTOM SAMPLES, MURRAY ISLAND, THE BAHAMAS, AND FLORIDA. 245 



areas where silt is being deposited, for there Sarcophyton and Xenia grow 

 plentifully. 



Echinoids. — Very few echinoid fragments were observed. 



Mollusca. — This is another important group, as it contributes a large 

 proportion of the bottom material. 



Bryozoa and Crustacea. — Some fragments of barnacle plates, ostracods, 

 and other Crustacea were found, but no bryozoa were recognized. 



The four groups of organisms which are most important are, named in 

 systematic order: (i) coralline algae; (2) foraminifera; (3) madreporarian 

 corals; (4) mollusca. 



DISTRIBUTING AND SORTING AGENTS. 



The following statement is quoted from Dr. Mayer's article (page 8 of 

 this volume) : 



"The strong southeast trade-wind, which prevails for about eight months of 

 the year, causes the ocean water on the incoming tide to sheer near the middle of the 

 southeast side of Maer Island, the currents parting, the stronger going around the 

 southwestern and the weaker around the northern end of the island. The current 

 around the southwestern side is reinforced by that around Dowar Island and is thus 

 stronger than that around the northeastern end. The silt from Haddon and Hedley 

 brooks is thus carried around the southwestern end of the island and contributes to 

 form the sand dunes, which are about 20 feet high, and to cover partially and smother 

 the reef-flats at the western corner of Maer Island. (See map, plate 2, and fig. 10.) 

 Several smaller sand dunes on the northern corner of the island are also formed by 

 the weaker northeasterly currents, and thus the northwest side of the island is 

 concave and lined throughout by a sand beach formed of volcanic and calcareous 

 fragments. It is interesting to observe that the sand derived from these currents 

 is tending to change the original oval shape of the island into a crescent, reminding 

 one of the manner in which an atoll islet acquires its typical crescentic shape, as 

 shown by Guppy, Hedley and Taylor, Wood Jones, and Vaughan. The outflowing 

 currents due to the falling tide are not competent to offset this eff^ect, for they must 

 make their v/ay against the prevailing southeast wind. At the Murray Islands the 

 tide rises between 7 and 8 feet, thus producing spring tide currents of nearly 4 knots 

 an hour around the southern end and a flow of about half that rate around the 

 northern end of the island." 



As will later be shown, there is very little fine material in the samples 

 from the southeast reef of Murray Islands and the proportion there is essen- 

 tially the same as in the samples taken from behind the reef off Cocoanut 

 Point, Andros Island, Bahamas. In both instances, in my opinion, the 

 explanation of the small percentage of particles of silt and clay size is attrib- 

 utable to outwash by currents and not to submarine solution. 



